1. Field of the Invention
The present invention relates to an apparatus and a method of quickly generating entanglement between distant sites before occurrence of decoherence to enable quantum communication such as quantum teleportation or quantum cryptography between distant sites.
2. Description of the Related Art
In conventionally proposed quantum communication such as quantum teleportation or quantum cryptography, two light beams in an entanglement state (quantum mechanically entangled state) generated in one place are transmitted to both the sender and the receiver so as to perform communication by using a quantum correlation existing between the two light beams (A. Furusawa et al., Science, 282, 06, (1998); A. K. Ekert, Phys. Rev. Lett., 67(6), 661(1991)). However, since a quantum state cannot be duplicated, it cannot be amplified or relayed during transmission. Therefore, the communication distance is limited below the attenuation distance of light. In quantum cryptography using no entanglement (C. H. Bennett et al., Scientific American, October, (1992)) as well, the communication distance cannot be increased by general amplification and relaying. This incapability of increasing the communication distance raises a common problem in putting the quantum communication into practice.
Recently, L.-M. Duan et al. has proposed a method of entangling distant atomic gas ensembles by connecting them by light (L.-M. Duan et al., Nature, 414, 413(2001)).
In this method, two atomic ensembles (A and B) at distant sites are irradiated with laser light, and photons generated from the respective ensembles are detected by a special method, thereby entangling the ensembles A and B. Pairs of atomic ensembles entangled with each other are generated in this manner. Here, a pair of atomic ensembles is represented by A(u) and B(u) (where u=1, 2, 3, . . . ). Thereafter, light is applied to one atomic ensemble of each pair (e.g., B(1) and A(2) of pairs of A(1) and B(1) and A(2) and B(2)), and photons generated from each atomic ensemble are detected by a special method, thereby newly entangling the other atomic ensemble of each pair (i.e., A(1) and B(2)).
If entanglement connections generated between two such atomic ensembles are further connected to each other, there is a possibility that entanglement generation can be done over a distance longer than the attenuation distance of light. In addition, since the quantum state of an atomic ensemble is used instead of that of a single photon or atom, the communication is robust against noise.
In increasing the communication distance by connecting such entangled ensembles, the maximum distance of quantum communication is finally determined by how quickly connections in many stages can be made within a time during which the overall coherency is kept. Forming such a quantum communication means by using a solid material will provide an advantage in realizing an easy-to-handle, compact element. In this case, since a decoherence time of a solid material is generally short, it is indispensable to shorten a time for attaining entanglement connection.
Conventionally, however, there have been no known apparatus or method that can efficiently generate entanglement between distant sites by quickly irradiating necessary sites with light necessary for entanglement connection without using any time-consuming mechanical driver.